1. Field of the Invention
This invention relates generally to time domain reflectometry systems, and more particularly to time domain reflectometry systems utilizing sampling techniques to detect leaks.
2. Description of the Prior Art
Various leak detection systems employing reflectometry techniques are known. In one such system, a pulse is sent down a transmission line placed in proximity to a pipe being monitored. Reflections of the pulses are monitored, and threshold values are established above and below the reflected waveforms. Upon the occurrence of a leak or break in the pipe, or a break in the cable, the characteristic impedance of the line changes, thus resulting in a change in the waveform of the reflection. If the change in impedance is sufficient to cause the waveform to cross either the upper or lower threshold, a leak or break is indicated.
Other systems for detecting leaks or analyzing transmission lines for the location of mismatches are also known. Typical examples of such systems are shown in U.S. Pat. Nos. 3,600,674, 3,981,181, 4,095,174, 4,491,782, 4,538,103 and 4,630,228. The above-mentioned patents disclose various ways of detecting leaks or impedance changes in cables using various analog and digital techniques. These systems include analog systems that compare an analog reflected waveform with predetermined thresholds, systems that utilize radar techniques, systems that detect changes in the distributed capacitance in a plurality of cables scanned in sequence, systems that digitize the waveform and compare the digitized waveform with a predetermined threshold, and systems that Fourier analyze the reflected waveform.
While these systems do provide a way to detect leaks or transmission line discontinuities, the analog systems are subject to drift. In addition, systems utilizing fixed thresholds to generate an alarm necessarily require a compromise between sensitivity and the ability to accommodate cable having a nonuniform characteristic impedance over their length. Also, many of the prior art systems are capable of only detecting a single leak in the system, i.e., they are capable of detecting the closest leak, but are not able to "see through" the closest leak to determine the existence of leaks farther down the line. In addition, many of the prior art systems require the use of complex and expensive high speed or high frequency circuitry to achieve the desired function.